Observations of the seasonal dependence of the thermal plasma density in the Southern Hemisphere auroral zone and polar cap at 1 R_E

Synoptic maps of the thermal plasma density in the auroral zone and polar cap around 1 R_E altitude are produced from measurements of Polar spacecraft floating potential during 1 year of perigee passes. The densities are accurate to around a factor of 2 in this region of space. These measurements, which provide the first comprehensive maps of the thermal plasma density over the polar cap and auroral oval, show clear variations in plasma density due to solar illumination of the ionosphere. Number density increases by a factor of 5 for illuminated verses nonilluminated ionospheric conditions over the entire auroral oval and polar cap. The maps delineate the global extent of the large-scale auroral density cavity near 70° ± 5° invariant latitude. These maps further show that the depth of the density cavity is strongly influenced by solar illumination. The minimum average density (∼0.1 cm^-3) occurs in darkness near 1900 magnetic local time.